The spectrum of excited nucleon states reflects the dynamics of QCD in the non-perturbative regime. It has been studied for many years using pion beams. However, the spectrum of known nucleon resonances is in conflict with predictions from quark models [1]}, [2]}. Most obvious is the missing resonance problem: the fact that more states are predicted by models at higher masses than have been observed experimentally. But also the ordering of excited states with positive and negative parity is partly in disagreement with experiments, the most prominent example being the \(N(1440)\,1/2^+\)  which is predicted by most quark models to be heavier than the \(N(1535)\,1/2^-\) . QCD calculations on the lattice [3]}, though using unphysically large quark masses, yield a similar pattern as the non-relativistic quark model. Measuring the properties of the known resonances more precisely and searching for new resonances is essential to understanding the discrepancies between theory and experiment.
